In popular music, a side project is a project undertaken by one or more people already known for their involvement in another band. It can also be an artist or a band temporarily switching to a different style.

Usually these projects emphasize a different aspect of that person's or that band's musical interests that they feel they cannot explore within the boundaries established by their main project. Side projects can later become full-time endeavours, but should not be confused with quitting a band for a solo career or another band. Peter Hartlaub of San Francisco Chronicle called the solo side project "the biggest longshot bet in mainstream music".[1]

The New York Times described the side project as "a break from the other band members, a chance to toy with different genres and recording methods, a fling with no long-term commitment".[2] There can be aesthetic reasons to pursue side projects,[3] and side projects can have the benefit of protecting indie credibility.[3]

"Side project" may also refer to pursuits of famous individuals outside of their primary fields. For instance, Wicked Wisdom is a "side project" of actress Jada Pinkett Smith. Side projects often occur at a crossroads of a celebrity's career.

A side project can also be a band that takes on an "alter ego", usually in order to play a different style of music that their fans are not used to. An example of this type of side project would be the band Weezer, who is also the Nirvana cover band Goat Punishment.

Another example of a side project is when in 1990, Jesper Strömblad of Ceremonial Oath decided to start a side project known as In Flames because he wanted to start a new project that combined Iron Maiden and death metal all together which he said he had never heard of before, the side project then grew into a band in 1995 and is now one of the most influential Metal bands of all time, and still together without Jesper.